Error (500)

exec(): Cannot execute a blank command

Call stack
# Function Location
0 !! exec(): Cannot execute a blank command !! ROOT/core/components/com_newsletter/site/controllers/newsletters.php:220
1 Hubzero\Base\Application->run() ROOT/index.php:83
2 Hubzero\Base\Stack->then() ROOT/core/libraries/Hubzero/Base/Application.php:432
3 Bootstrap\Site\Providers\DocumentServiceProvider->handle() ROOT/core/libraries/Hubzero/Base/Stack.php:106
4 Hubzero\Base\Middleware->next() ROOT/core/bootstrap/Site/Providers/DocumentServiceProvider.php:60
5 Hubzero\Base\Stack->next() ROOT/core/libraries/Hubzero/Base/Middleware.php:35
6 Bootstrap\Site\Providers\ComponentServiceProvider->handle() ROOT/core/libraries/Hubzero/Base/Stack.php:126
7 Hubzero\Component\Loader->render() ROOT/core/bootstrap/Site/Providers/ComponentServiceProvider.php:50
8 Hubzero\Component\Loader->execute() ROOT/core/libraries/Hubzero/Component/Loader.php:252
9 Hubzero\Component\Loader->executePath() ROOT/core/libraries/Hubzero/Component/Loader.php:273
10 require_once() ROOT/core/libraries/Hubzero/Component/Loader.php:298
11 Components\Newsletter\Site\Controllers\Newsletters->execute() ROOT/core/components/com_newsletter/site/newsletter.php:27
12 Hubzero\Component\SiteController->execute() ROOT/core/components/com_newsletter/site/controllers/newsletters.php:35
13 Components\Newsletter\Site\Controllers\Newsletters->outputTask() ROOT/core/libraries/Hubzero/Component/SiteController.php:339
14 exec() ROOT/core/components/com_newsletter/site/controllers/newsletters.php:220
15 Hubzero\Error\Handler->handleError()